1. Field of the Invention
The present invention relates to a quartz crystal oscillator in which phase noise is reduced by inserting a crystal resonator in a feedback loop of an amplifier for oscillation, and more particularly to a crystal oscillator that reliably maintains oscillation.
2. Description of the Related Art
Crystal oscillators feature a high degree of stability of oscillation frequency and are therefore frequently used in high-performance radio equipment. In these crystal oscillators, Japanese Patent Laid-open Application No. 9-153740 (JP-A-H9-153740), for example, discloses a crystal oscillator in which a crystal resonator is arranged in the output portion of an oscillation circuit separate from the quartz crystal unit for oscillation. The crystal oscillator having a separate crystal resonator can both increase frequency stability and reduce phase noise.
FIG. 1 shows an example of the configuration of a conventional crystal oscillator that is provided with a crystal unit and a crystal resonator. This crystal oscillator is basically made up from resonance circuit 1 and amplifier 2 for oscillation.
Resonance circuit 1 is composed of crystal unit 3 as an inductive component, and split capacitors 4a and 4b. Capacitors 4a and 4b are connected together in a series, and crystal unit 3 is further connected in parallel to the serially connected pair of capacitors 4a and 4b. 
Amplifier 2 is provided with transistor Q for oscillation and feeds back and amplifies an oscillation frequency component that depends on resonance circuit 1. In amplifier 2, the base of transistor Q is connected to the connection node between capacitor 4a and crystal unit 3, and the collector of transistor Q is connected to power supply Vcc by way of collector resistor R4. The collector is also connected to output terminal Vout. The emitter of transistor Q is connected to one end of load resistor R3. The other end of load resistor R3 is connected both to ground and to the connection node between capacitor 4b and crystal unit 3. Output line 5 that makes up one portion of the feedback loop is provided such that the connection node between capacitors 4a and 4b is connected with the emitter of transistor Q. Crystal resonator 6 is inserted in output line 5. Bias resistors R1 and R2 are provided for applying a bias voltage to the base of transistor Q.
Both of crystal unit 3 and crystal resonator 6 are configured from crystal blanks cut at the same cut angle from a quartz crystal block. More specifically, crystal unit 3 and crystal resonator 6 are constituted from, for example, an AT-cut quartz crystal blank. The resonant frequency of crystal resonator 6 is then set to generally match the oscillation frequency of the crystal oscillator. In this crystal oscillator, only the fundamental wave component of oscillation frequency f passes through crystal resonator 6, whereby the output signal is a narrow band and the phase noise in the output signal can be reduced. In other words, crystal resonator 6 is used as a filter for removing the spurious component in the oscillation output and extracting only the fundamental wave component. Further, output terminal Vout may be connected to the emitter of transistor Q.
To attain a further stabilization of the oscillation frequency in this type of crystal oscillator, crystal unit 3 is normally accommodated within a thermostatic oven and caused to operate in an environment in which the temperature is fixed.
Curve A in FIG. 2 shows the relation between temperature and the frequency deviation Δf/f where f is the nominal oscillation frequency of the crystal unit, and Δf is the difference of the actual oscillation frequency from the nominal oscillation frequency f. Typically, the frequency deviation exhibits change that can be represented by a cubic function curve with respect to temperature. As can be seen from this graph, the crystal unit has a minimum oscillation frequency in the vicinity of 70° C., and the temperature in the thermostatic oven that accommodates crystal unit 3 is normally set to approximately 70° C.
However, because the crystal oscillator of the above-described configuration uses a thermostatic oven, some time period is required for the temperature in the thermostatic oven to reach, for example, 70° C. from the introduction of the power supply, and the problem therefore occurs that the start-up characteristics are poor during this interval due to changes in the oscillation frequency and instability of oscillation.
Crystal resonator 6 also has a frequency-temperature characteristic that exhibits a cubic function curve similar to that of the crystal unit. The crystal resonator is arranged outside the thermostatic oven, and the temperature of the crystal resonator is therefore normal temperature while the temperature of the crystal unit is, for example, 70° C. The crystal resonator is designed such that the resonant frequency of the crystal resonator will match the fundamental wave component of the oscillation frequency that results from the crystal unit, but if the temperature of the crystal unit is close to normal temperature immediately following the introduction of the power supply, the oscillation frequency resulting from crystal unit will vary widely from the resonant frequency of the crystal resonator, thus raising the problem that the fundamental wave component of the oscillation frequency will not pass through the crystal resonator and the circuit will not oscillate.
Accommodating the crystal resonator in a thermostatic oven can also be considered, but adopting such an approach would necessitate a larger thermostatic oven and would both increase power consumption and impede miniaturization of the crystal oscillator.
In response to this problem, the inventors of the present invention proposed a crystal oscillator in US 2005/0285690 A1 in which a temperature compensation mechanism is connected to both the crystal unit and crystal resonator to both eliminate the need for a thermostatic oven and prevent halts in oscillation caused by temperature-changes. As the temperature compensation mechanism used in this case, there is an indirect type in which a voltage-variable capacitance element is connected to each of a crystal unit and crystal resonator to apply a compensation voltage from a compensating voltage generation circuit to the voltage-variable capacitance element, and a direct type in which the temperature compensation circuit is directly connected to each of the crystal unit and crystal resonator.
FIG. 3A shows the configuration of a crystal oscillator that uses the temperature compensation mechanism of the indirect type. This crystal oscillator is of a configuration in which, in the circuit shown in FIG. 1, voltage-variable capacitance element 7a is inserted between crystal unit 3 and ground, and voltage-variable capacitance element 7b is inserted in output line 5 between the node at which split capacitors 4a and 4b are connected together and crystal resonator 6. Resistor R7 is inserted between ground and the connection node between voltage-variable capacitance element 7b and crystal resonator 6. In addition, compensation voltage generation circuit (TCN) 8 is provided for generating a compensating voltage according to the ambient temperature, and the compensating voltage is applied to voltage-variable capacitance elements 7a and 7b by way of high-frequency blocking resistors R5 and R6, respectively. As voltage-variable capacitance elements 7a and 7b, variable capacitance diodes may be used.
In the crystal oscillator shown in FIG. 3A, a compensation voltage is applied to voltage-variable capacitance elements 7a and 7b, whereby the load capacitance as seen from crystal unit 3 and crystal resonator 6 changes depending on the compensating voltage, and the resonant frequency of resonance circuit 1 that contains crystal unit 3 and the resonant frequency of crystal resonator 6 change according to the compensating voltage. By here making the temperature characteristic of the compensating voltage the inverse characteristic, as shown by curve B of FIG. 2, of the frequency-temperature characteristic of the crystal unit and crystal resonator, changes in the resonant frequency caused by the compensating voltage will cancel the changes in the resonant frequency caused by the ambient temperature, and the oscillation frequency of the crystal oscillator is thus kept uniform regardless of the ambient temperature.
FIG. 3B shows the temperature compensation mechanism of the direct type. When the temperature compensation mechanism of the direct type is used, temperature compensation circuit 9 made up from thermistors Rt1 and Rt2 and capacitors C1 and C2 is used, and this temperature compensation circuit 9 is connected in a series to each of crystal unit 3 and crystal resonator 6. In temperature compensation circuit 9, thermistor Rt1 and capacitor C1 are connected in parallel to form a high-temperature compensation circuit, thermistor Rt2 and capacitor C2 are connected in parallel to form a low-temperature compensation circuit, and the high-temperature compensation circuit and low-temperature compensation circuit are then connected in a series. In the case of using this temperature compensation circuit 9 as well, the frequency-temperature characteristics of crystal unit 3 and crystal resonator 6 are compensated, and the oscillation frequency of the crystal oscillator is kept uniform regardless of the ambient temperature.
Nevertheless, even in a crystal oscillator in which a temperature compensation mechanism has been provided to eliminate the need for a thermostatic oven, and moreover, to prevent oscillation halts due to temperature changes, oscillation may still be interrupted due to changes in the power supply voltage and the like. When the power supply voltage to oscillation circuit 1 changes, the oscillation frequency generated by crystal unit 3 changes slightly, but the resonance frequency of crystal resonator 6 remains almost unaffected by the power supply voltage. Thus, when the oscillation frequency generated by crystal unit 3 changes due to fluctuation in the power supply voltage, there arises the possibility that, as in the previously described case, the oscillation signal will not be able to pass through crystal resonator 6 and oscillation will halt. This problem of interruptions in oscillation can occur regardless of the provision or lack of a thermostatic-oven or temperature compensation mechanism.
Even if the resonant frequency of crystal resonator 6 is set to coincide with the oscillation frequency produced by crystal unit 3, the occurrence of a change in only the oscillation frequency produced by crystal unit 3 due to any reason will prevent the oscillation frequency component from passing through crystal resonator 6, raising the problem of interruptions in oscillation. This problem occurs because crystal unit 3 and crystal resonator 6 both have high resonance acutance (i.e., Q value) and the pass frequency range of crystal resonator 6 has a narrow pass bandwidth in, for example, the 3 dB attenuation band.